A bicycloalkane is a cycloalkane whose molecule contains two rings. The number of rings in a cycloalkane is defined as the minimum number of bonds that must be broken in order to convert the molecule into an open-chain fragment.
The minimum number of bonds that must be broken in order to convert Decalin into an open-chain fragment is two. Decalin, therefore, is a bicycloalkane.
see also monocycloalkane, polycycloalkane